In the specification that follows the term “NAC” is to be interpreted as meaning non-acid chemicals selected from the group including: acetone and higher ketones, methanol, ethanol, propanol and higher alcohols, i.e. oxygenated hydrocarbons excluding acids.
The term “hydrocarbons” is to be interpreted as hydrocarbons normally not soluble in water, such as, for example, paraffins and olefins.
The water-rich stream produced in a Fischer-Tropsch (FT) Synthesis unit contains various oxygenates such as alcohols, aldehydes, ketones, carboxylic acids, and the like, that are products of the FT synthesis reaction. These compounds are found (in part) in the water stream due to their partial or full solubility in water.
A distillation column is required to remove the non-acid chemicals (NAC's) such as alcohols, ketones, aldehydes, and other non-acid compounds from the water-rich stream, so that the upgraded water can be treated further before it is released into the environment. The NAC-rich stream from the distillation column can be worked up further into products or may find alternative applications.
The fractionation between NAC's and water in the distillation column, which is commonly referred to as the Reaction Water Distillation (RWD) Column, is complicated by the extreme non-ideal behaviour between water and heavier organics present in the water stream, notably the C4 and heavier alcohols. This non-ideality makes these compounds easy to strip from the water-rich liquid phase below the feed tray, which is the purpose of the column. However, above the feed tray, as the water content of the liquid in the column decreases, the heavier alcohols become less volatile and tend to condense again.
The result is a tendency of the heavy alcohols to accumulate in the column, eventually to the point where a second liquid phase forms. This oxygenated hydrocarbon phase contains much more of the heavy alcohols and much less water than does the first phase. If the second liquid phase is left in the column, it provides a low-volatility path for heavy alcohols to migrate downward, until revaporized by rising vapour in the column. This results in circulation of oxygenated hydrocarbons such as, for example, heavy alcohols within the column, poor liquid distribution on the trays and eventual breakthrough of heavy material in the bottoms. Such a breakthrough will cause the bottom product to violate specifications on the bottom product and could cause problems in the downstream water treatment facility due to contamination.
Therefore, the oxygenated hydrocarbon phase formed inside the column is normally removed via a relatively small vapour stream in the bottom section, typically a few trays above the reboiler, of the column. This vapour stream is then condensed and separated into two phases. The water-rich stream is sent back to the column by either mixing it with the feed to the column or by feeding it to the column on its own. The oxygenated hydrocarbon phase is typically mixed with the overhead stream for further processing.
This vapour draw-off is however not sufficient to remove the oxygenated hydrocarbon phase to such an extent that it would not appear in the column any more. The vapour draw is only able to remove enough of the oxygenated hydrocarbon phase to inhibit breakthrough to the bottom product. A large circulation of the organic phase therefore still takes place within the column, making it a relatively inefficient way of separating the chemicals from the water.